Liquid crystal displays are now used for various applications such as notebook-size PC, mobile terminals, desktop monitors, digital cameras, and the like by making use of the characteristics of the liquid crystal displays, such as light weight, thinness, low power consumption, and the like. In order to further decrease the power consumption of a liquid crystal display using a backlight, the utilization efficiency of the backlight must be improved. Therefore, a color filter is required to have improved transmittance. Although the transmittance of the color filter is being improved every year, a significant decrease in power consumption is not expected from improvement in transmittance.
In recent years, the development of a reflective liquid crystal display not requiring a backlight source having high power consumption has been advanced, and it has been reported that the power consumption can be decreased to about 1/7 of that of a transmissive liquid crystal display (Nikkei Macrodevice, separate volume, Flat Panel Display, 1998, P. 126).
A reflective liquid crystal display has the advantage of low power consumption and excellent outdoor visibility. However, in a place where sufficient environment light cannot be secured, a display becomes dark, thereby causing. the problem of significantly deteriorating visibility. In order to make a display visible in a place where environmental light has low strength, two systems each comprising-a light source have been designed. One of the systems is (1) a liquid crystal display, i.e., a so-called transflective liquid crystal display, comprising a backlight as a light source and a reflection layer provided in each pixel and having a notch so that the a part of the reflection layer is used for a transmissive display system, and a part of the reflection layer is used for a reflective display system (refer to documents such as Fine Process Technology Japan '99, Expert Technology Seminar Text A5). The other system is (2) a reflective liquid crystal display comprising a front light.
As each of a backlight source and a front light source used for mobile terminals, a three-peak type fluorescent lamp or a white LED (light emitting diode) is used. The three-peak type fluorescent lamp is advantageous from the viewpoint of power consumption, and this fluorescent lamp is known to improve the color reproducibility of transmitted color light. Thus, the three-peak fluorescent lamp is used for relatively large mobile terminals such as a mobile PC, PDA, and the like. On the other hand, a white LED is advantageous for a smaller size and thickness and is used for small mobile terminals such as a cellular phone and the like.
White LEDs are divided into a two-peak type and a three-peak type according to the spectral shapes. The two-peak type white LED uses a combination of a blue LED and a phosphor, for obtaining a white color (FIG. 5). On the other hand, the three-peak type white LED uses a combination of an ultraviolet LED and red, green and blue phosphors (FIG. 1), or a combination of LEDs of the three colors, i.e., red, green and blue (FIG. 2), for obtaining a white color. The two-peak type white LED has been used as an option of the white LED light source so far (Nikkei Electronics, 2002, No. 2-25).
A transflective display comprising a backlight comprises picture elements in each of which a transmissive display using backlight and a reflective display using environmental light coexist, and thus a display with excellent visibility can be achieved regardless of the strength of environmental light. However, in the use of such a conventional color filter as shown in FIG. 6, i.e., a color filter in which a reflective region and a transmissive region are not provided in each picture element to cause uniform color characteristics within each picture element, a problem occurs in producing a vivid transmissive display. More specifically, when the color vividness (color purity) of transmitted color light is improved, the color purity of reflected color light is also further increased to significantly decrease brightness having a trade-off relation to color purity. Therefore, sufficient visibility cannot be obtained for reflective display. This problem is due to the fact that in the transmissive display, backlight is transmitted once through the color filter, while in the reflective display, environmental light is transmitted through the color filter two times-including the time of incidence and the time of reflection. In the transmissive display, the backlight is used as a light source, while in the reflective display, natural light is used as a light source. Therefore, the transmissive display and reflective display are different in not only color purity but also color tone. This is due to the fact that natural light has a continuous spectrum like the spectrum of light source D65 shown in FIG. 13, while the backlight source has spectral peaks at characteristic wavelengths, as shown in FIGS. 1 to 5.
A method for solving the above problem is a method in which a transparent resin layer is formed in each reflective region to thin a colored layer in each reflective region, thereby improving the brightness of a reflective display. This method is a so-called thickness controlling method and disclosed in Japanese Unexamined Patent Application Publication No. 2001-33778. FIG. 7 schematically shows a cross-section of a conventional color filter for a transflective liquid crystal display. A transparent resin layer 3 is formed in each reflective region 6 to decrease the thickness of a colored layer 5 in each reflective region 6, as compared with the thickness of the colored layer 5 in each transmissive region 7. In order that the colored layer in each reflective region has the same degree of brightness as that of the colored layer in each transmissive region, the thickness of the colored layer in each reflective region must be set to ½ or less of the thickness of the colored layer in each transmissive region. On the other hand, an increase in the degree of thinning of the colored layer in each reflective region causes large variations in the thickness, i.e., large variations in display colors, thereby causing a manufacture problem such as a reduction in yield, or the like. In consideration of processability and improvement in the brightness of the reflective display, the thickness of the colored layer in each reflective region must be set to about ½ to ⅖ of the thickness of the colored layer in each transmissive region. In this method for improving the color reproducibility in the transmissive display, with the above-described degree of thinning, sufficient brightness cannot be obtained in the reflective display, thereby causing the problem of failing to satisfy both a vivid transmissive display and a bright reflective display. Also, the problem in which the transmissive display and the reflective display have different color tones cannot be solved only by changing the thickness.
In the use of a color filter in which a transmissive region and a reflective region is coated separately, as shown in FIG. 8, color purity and brightness can be freely changed, thereby achieving a transmissive display color and brightness, and a reflective display color and brightness adequate for the purpose. In this method (six-color coating method), the color layers of the reflective regions are independent of those of the transmissive regions, and thus the reflective display with sufficient brightness can be achieved even when the color reproducibility of the transmissive display is increased. However, in a photolithography method which is a main stream at present, coating of a coloring agent and photolithography process are performed two times or more for forming picture elements of one color, and thus two lithography processes for each color, i.e., a total of six photolithography processes, are required for forming the picture elements of the three colors including red, green and blue, thereby causing the problem of increasing the manufacturing cost. When a coloring agent is coated in the transmissive regions (or the reflective regions), and then a coloring agent is coated in the reflective regions (or the transmissive regions), from the viewpoint of production, it is difficult to coat the coloring agents in such a manner that no space occurs between the transmissive regions and the reflective regions, and the coloring agents do not overlap with each other. Therefore, there is the possibility that the product yield is decreased to increase the manufacturing cost of the color filter. When spaces occur between the reflective regions and the transmissive regions, light leaks from the spaces to decrease the image quality of a liquid crystal display. On the other hand, when the coloring agents overlap with each other, the color only at the boundaries is increased and is possibly recognized as ununiformity on a screen. Furthermore, the cell gap of the liquid crystal display becomes defective. Namely, the yield of the liquid crystal display deteriorates to possibly increase the manufacturing cost of the liquid crystal display.
As a method capable of a transmissive display and reflective display with high color reproducibility, a method comprising forming an aperture in each reflective region to improve the brightness of the reflective display, i.e., an area controlling method, is disclosed in Japanese Unexamined Patent Application Publication No. 2000-111902. FIG. 9 schematically shows a cross-section of a conventionally known color filter for a transflective liquid crystal display having the apertures. In this case, only three times of photolithography processes are performed to permit the manufacture of a color filter at low cost. However, this method decreases the color purity-reflectance characteristics of the reflective display, as compared with the method in which the transmissive regions and the reflective regions are coated separately. Therefore, this method has the problem of failing to satisfy both color vividness and sufficient brightness. Particularly, when the color reproducibility is increased in the transmissive display and the reflective display, the brightness of the reflective display is decreased to cause the insufficient performance as a liquid crystal display.
In a conventional transflective liquid crystal display such as a mobile terminal or the like a two-peak type LED light source or three-peak type fluorescent lamp is used. However, a combination with a conventionally known low-cost color filer for a transflective liquid crystal display has the problem of failing to achieve a level in which both the high color reproducibility of the transmissive display and the sufficient brightness of the reflective display are satisfied.